Homer goes to Moe's Tavern with a new suit, but when he tries to pay for the beer, he finds out that his pocket has much old stuff, including an old roll of film, Moe tells Homer that his bar is legally a darkroom and Homer decides to develop the film. After bringing the family, they realize why the film was never developed: because it was full of photos of young Bart and Lisa fighting. The kids get curious about how they stopped fighting each other all the time, and Marge tells them a story, that leads to a flashback.
The flashback takes place six years ago in 2009, when Bart was 4 years old and Lisa was 2. They went to a library to hear a storyteller, but the siblings start hitting each other with books and were expelled from the library. Later that night, Bart is scared because of his clown bed, and he does a doodle about the bed, but Lisa takes his pencil claiming he stole it, and shows him that she could write his name better than Bart himself. Bart gets angry and starts hitting her with a toy, but Homer sees that and strangles him. Bart hits Homer in the head with a lampshade, making Marge disappointed. Marge was so frustrated about the kids that she starts having nightmares, and they decide to take them to a psychologist. The doctor tells them that one of their kids is smart and good (Lisa) and the other is dim and evil (Bart). Back at the house, Ned Flanders decides to help them, arranging for Grandma Flanders to babysit the kids for them to enjoy a day without the kids, but they enjoy their day at the house, causing them to be late for brunch with the Flanders. At the Flanders house, Grandma Flanders appears to die, leaving the kids unsupervised. Bart and Lisa try to go back home, but the door is locked. They hear an ice cream truck, Bart takes his tricycle and Lisa takes her pedal car to buy ice cream, but they get lost in the city.
Later, Ned finds his Grandma unconscious and the Simpsons kids gone, much to Homer and Marge's despair. In the city, Bart goes into an alley, where he finds Kearney, Dolph and Jimbo. The bullies take his tricycle, but Lisa starts crying and they decide to leave them alone. At this moment, Bart realizes Lisa was smart and they make a good team. Then, they go to the Retirement Castle, where grandpa takes care of them. Homer and Marge tries to find the kids desperately, and they eventually found them, on top of the Tire Fire. Homer tries to bend a tree to save them, but the branch he was holding broke, catapulting the kids back to the Simpsons' house into the clown bed, which breaks apart. Bart hears another ice cream truck, but Lisa holds him back. Back to the present time, Marge tells that since that day, they got along, and they were able to have Maggie. In the ending of the episode, Dr. Hibbert tells Ned his grandmother survived, and Moe calls Homer "Father of the Year."